In general, materials which have been used for rolling bearings include SUJ2 for bearing steels, SUS 440C and 13Cr martensitic stainless steels for stainless steels, and steel products corresponding to SCR 420 and SCM 420 for case hardening steels. The rolling bearings are used while being subjected to repeated shear stress under high contact pressure. In order to ensure rolling fatigue strength resisting the shear stress, therefore, hardening and tempering are performed to bearing steels, hardening, subzero treatment and tempering to stainless steels, and hardening and tempering to case hardening steels after carburizing or carbonitriding to realize a hardness of 58 to 64 in HRC.
However, these rolling bearings are used under a great variety of circumstances, and the use of ball bearing steels causes the possibility of early rusting by salt damages in regions adjacent to sea, invasion of water or sea water, or exposure or use under corrosive circumstances such as wetted conditions and the like.
Then, in the rolling bearings used under corrosive circumstances or particularly requiring the prevention of rusting, high carbon Cr martensitic SUS 440C has hitherto been used as stainless ball bearing steels excellent in corrosion resistance and having a hardness of 58 or more in HRC which is necessary for the bearings.
Further, the rolling bearings used in various spindles, various spindle motors and swing arms of HDD devices are required to have excellent rotational and acoustic performances and resistance to fretting, and to be small in torque fluctuations. However, the conventional stainless steels contain a number of coarse eutectic carbides having a size of more than 10 .mu.m, so that it is difficult to obtain desired working accuracy. For example, the acoustic performance thereof is liable to be inferior to those of the rolling bearings made of ball bearing steels.
Then, in the spindle motors particularly used for driving magnetic discs for rotation and requiring sufficient rotational and acoustic performances, the rolling bearings made of ball bearing steels are used in many cases for this reason. In contrast, the rolling bearings for swing arms used for driving the swing arms performing access positioning to effective areas of the magnetic discs are used under swinging conditions. It is therefore difficult to form oil films between the rolling elements and races, so that torque fluctuations and torque spikes are developed by fretting wear to cause harm to the read function of the HDD devices in some cases.
Accordingly, high carbon Cr stainless steels good in resistance to fretting are used in many cases in the rolling bearings for swing arms, and in recent years, 13Cr martensitic stainless steels improved in rotational and acoustic performances are often used as stainless ball bearing steels for the rolling bearings used in the swing arms of the HDD devices and the like.
On the other hand, in the case of linear guide bearing apparatuses comprising guide rails and sliders, or ball screw apparatuses comprising screw shafts and nuts, JIS-SUS 440C and 13Cr martensitic stainless steels (carbon contents 0.6% to 0.7%) and further precipitation hardening type stainless steels such as JIS-SUS 630 are used as corrosion-resistant stainless materials used therein.
Case of Rolling Bearings
In the high carbon Cr stainless steels as described above, when the content of C and Cr is increased, for example, when C is contained in an amount of more than 0.6% by weight, a number of coarse eutectic carbides having a size of more than 10 .mu.m are formed coupled with a large amount of Cr. These not only reduce the fatigue strength, toughness and resistance to corrosion of the rolling members, but also deteriorate workability such as the malleability and machinability.
Further, the presence of the coarse eutectic carbides sometimes adversely affects the acoustic performance of the rolling bearings. The acoustic performance indicates the degree of noise developed by signals generated in operation of the rolling bearings, and often causes a serious problem in relatively small-sized stainless steel rolling bearings used in precision instruments such as HDD devices which are apt to be easily damaged by vibration.
That is to say, the vibration developed in the bearings largely depends on the configurational accuracy of outer races, inner races or shaft elements, and rolling elements thereof. Accordingly, when materials containing the coarse eutectic carbides are used for the bearings, the coarse carbides inhibit the achievement of desired accuracy in finishing the bearings, and further, the difference in wear between grounds and the eutectic carbides arises also in rotation operation to cause a reduction in accuracy of roughness and the like. Furthermore, these eutectic carbide particles interfere with one another at their contact surfaces, resulting in increased noise.
As described above, the coarse eutectic carbides not only deteriorate the acoustic performance of the bearings, but also become sources of stress concentration to decrease the fatigue strength, and further to cause deterioration in toughness and resistance to corrosion. Accordingly, the high carbon Cr martensitic stainless steels such as SUS 440C not only have no sufficient resistance to corrosion and no mechanical strength, but also is extremely poor in acoustic performance, and is further high in cost. It has been therefore impossible to suitably use it for the rolling bearings used under corrosive circumstances or the rolling bearings used in various spindles, various spindle motors and the swing arms for the HDD devices.
Further, these rolling bearings are fixed with adhesives in many cases, and adhesion of rust preventive oil raises various problems. For example, the adhesive strength is decreased, or the rust preventive oil chemically reacts with the adhesives to contribute development of rust and further to generate out gas, which adheres to disc surfaces to reduce reliability of the HDD devices. Accordingly, the rolling bearings are completely degreased in many cases. It is therefore considered that stainless steels are better for the rolling bearings. However, stainless steels are high in cost compared with bearing steels such as SUJ2. Moreover, it contains a number of coarse eutectic carbides, although it is good in resistance to corrosion and resistance to fretting compared with SUJ2. It is therefore difficult to obtain target working accuracy. Further, these rolling bearings tends to be inferior to ones made of SUJ2 in acoustic performance, so that it is difficult to use them for the rolling bearings for spindle motors requiring high rotational and acoustic performances. Furthermore, with respect to the rolling bearings for various spindles, further improvements in resistance to fretting during conveyance and operation of the spindles have been desired.
JP-B-5-2734 (the term "JP-B" as used herein means an "examined Japanese patent publication") discloses martensitic stainless steel rolling bearings significantly improved in acoustic performance and fatigue strength in which the content of C and Cr is decreased to inhibit formation of eutectic carbides. However, they are inferior to SUJ2 in the size of carbides, and are not only poor in acoustic performance, but also take cost into no consideration. Further, this publication discloses no resistance to fretting which is a required characteristic for the rolling bearings for HDD swing arms at all, and further describes no workability at all.
Further, in JP-A-6-117439 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"), attempts have been made to form both or one of outer races and inner races of martensitic stainless steels, and to form rolling elements of ball bearing steels, thereby reducing cost and improving the acoustic performance. However, stainless steels used in races in this publication are the same as the martensitic stainless steels described in JP-B-5-2734. These stainless steels are inferior to ball bearing steels such as SUJ2 in acoustic performance, so that the ball bearings described in this publication are inferior to ball bearings in which inner races, outer races and rolling elements are all formed of ball bearing steels in acoustic performance. Similarly to JP-B-5-2734, this publication also discloses no resistance to fretting which is a required characteristic for the rolling bearings for HDD swing arms at all, and further describes no workability at all.
In the case where the resistance to corrosion is required, both the above-mentioned prior techniques do not have the sufficient measures necessarily.
Case of Linear Guide Bearing Apparatuses and Ball Screw Apparatuses
When parts used are in complicated deformed cross sectional form, like sliders and guides of linear guide devices, raw materials are deformedly cold drawn to a form as near to the form of finished articles as possible in many cases. Accordingly, stainless steels difficult to be worked are high in resistance to deformation and work hardening, so that they have the problem that the deformed drawing thereof is highly expensive and time-consuming, resulting in extremely high cost compared with case hardening steels such as SCM 420 and SCR 420 which are relatively good in workability and carbon steels for machine structural use such as S 45C and S 53C.
In particular, precipitation hardening type stainless steels such as SUS 630 are highly corrosion-resistant, but they have a hardness of about 33 to about 36 in HRC even in the solution-treated state. These stainless steels are therefore extremely poor in cold workability, resulting in a very high production cost of parts , so that they are not used so much except specific fields such as atomic power-related fields.
Then, 13Cr high carbon martensitic stainless steels have recently been used in which the content of C and Cr in SUS 440C difficult to be worked is decreased to improve cold deformedly drawing workability. However, even in this material, the improving effect is insufficient yet. Dies are early broken, or revisions become necessary. Thus, the problems are not sufficiently solved.
Further, the conventional high carbon martensitic stainless steels are work hardened to sometimes develop aging cracks vertically along a groove largest in the reduction ratio of area in deformedly drawing it. Special attention such as tempering immediately after drawing should be given.
Similarly to the rolling bearings, the linear guide apparatuses having the above-mentioned rolling elements are often used while being subjected to repeated shear stress under high contact pressure. In order to ensure rolling fatigue strength resisting the shear stress, therefore, tempering is performed after forming to increase the surface hardness in production thereof. As the conventional tempering methods, methods of conducting through hardening under vacuum and methods of conducting induction hardening are known. However, in the former, the whole parts are heated to high temperatures, followed by rapid cooling. Accordingly, more deformed or longer parts results in larger unbalance in cooling velocity of the whole parts, which causes large deformation and bend of the parts in some cases.
On the other hand, the latter induction hardening methods have the advantages that partial hardening can be easily performed by the design of hardening coils, that deformation, bend and twist are scarcely developed because of no necessity to apply heat to the whole parts, and that the deformation and the like are easily corrected, because non-hardened layers such as core portions are sufficiently soft. Accordingly, of the parts such as the rails and sliders constituting the linear guide devices, particularly, deformed or long ones are hardened by the latter induction hardening in many cases.
However, the conventional stainless steels are low in diffusion velocity of carbon compared with carbon steels for machine structural use, and further, coarse eutectic carbides formed in the course of solidification are difficult to be dissolved in grounds. It is therefore difficult to obtain the depth of hardened layers by heating for a short period of time according to induction hardening. When the deep depth of the hardened layers is intentionally obtained by high frequency output or hardening velocity, the excess dissolution of the carbides lowers the Ms point to inhibit the martensitic transformation. Accordingly, a large amount of the remaining untransformed austenite raise the problem of failure to obtain sufficient surface hardness or generation of unevenness in hardness. Further, the conventional stainless steels contain a number of coarse eutectic carbides independent of hardening methods, so that they act as sources of stress concentration to decrease the rolling fatigue strength, and further to have no sufficient resistance to corrosion.
JP-A-2-310342 and JP-A-3-138335 disclose martensitic stainless steels for cold forging and methods for producing them. These stainless steels contain not only the insufficient content of carbon contributing to solid solution or precipitation strengthening, but also no nitrogen having suitable influences on the rolling fatigue strength, wear resistance and resistance to corrosion. They are therefore insufficient for bearing application receiving repeated fatigue under high contact pressure.
Further, the linear guide devices and the ball screw apparatuses are accompanied by repeated reciprocating motion at short stroke, so that they require the resistance to fretting, similarly to the rolling bearings.
Case of Ones Used under Severe Conditions Such As Corrosive Conditions (Problems common to Rolling Bearings, Linear Guide Bearing Apparatuses and Ball Screw Apparatuses)
When the resistance to corrosion is particularly required for the above-mentioned rolling apparatuses such as the rolling bearings, the linear guide bearing apparatuses and the ball screw apparatuses, hard Cr plating, fluoride laident and other various surface coating treatments are performed on these stainless steels, bearing steels and further carburized or carbonitrided case hardening steels. However, the rolling bearings have the problem that they are damaged in the vicinities of surfaces thereof by fatigue and wear to easily separate off coatings discontinuous to base phases, resulting in failure to obtain sufficient durability.
On the other hand, German Patent No. 3,901,470 discloses that unprecedented highly corrosion-resistant and high functional martensitic stainless steels are obtained by substituting carbon by nitrogen. Such stainless steels have recently been presented in many literatures mainly in Europe (Proceeding of International Congress Stainless Steel, 42-46 (1996) and the like) (material name: Cronidur 30 or X30).
In general, addition of nitrogen to stainless steels are mainly conducted in austenitic stainless steels. Martensitic stainless steels have the problem that the solubility of nitrogen is low and therefore inclusion of a large amount of nitrogen induces the development of bubbles in the course of solidification to introduce a large number of pores into ingots, resulting in damage of the soundness of the materials. Accordingly, this technique has not hitherto been actively made, and has scarcely come in practice.
In contrast, the above-mentioned German Patent makes it possible to alloy more than 0.3% of nitrogen by performing steel production in a pressure vessel under an atmosphere of nitrogen having a pressure of tens of atmospheres. However, in other words, the steel production in a pressure vessel is necessary, and the course of steel production becomes special. There is therefore the problem that an increase in cost is unavoidable in terms of investment in equipment and productivity.
Further, materials used for applications of aircraft and corrosion-resistant bearings used at relatively high temperatures require not only excellent resistance to corrosion, but also excellent durability at high temperatures. However, as to the materials described in the above-mentioned German Patent, there is still room for improvement in this respect.